Over time electron guns used in high resolution color CRTs have evolved from the individual type of main lens design to the common lens type design. The former enploys three separate electro-optic lenses, one for each of the three inline electron beams. This type of electron gun suffers from a spatial limitation which gives rise to high spherical aberration and generally poor electron beam spot resolution at high beam current. In the so-called "common lens" design, the three electron beams are directed through a shared aperture as well as through a shared focus region. By increasing the cross sectional size of the electro-optic lens through which the electron beams are directed (without increasing the diameter of the CRTs neck portion), a substantial reduction in spherical aberration, particularly in the horizontal direction, is realized. A single, shared aperture in the common lens is generally elongated in the horizontal direction, somewhat enlarged in the vertical direction, and may assume various shapes such as that of a racetrack, dog bone, or chain-link configuration.
Referring to FIG. 1, there is shown a partially cutaway perspective view of a prior art electron gun 10. The upper right portion of each grid as the electron gun 10 is viewed in tile direction of the CRT's display screen is removed in the figure in order to illustrate the beam passing apertures in these grids. A side elevation view of the electron gun 10 is shown in FIG. 2. Electron gun 10 includes G1 control and G2 screen grids each having three respective inline, beam passing apertures 30a, 30b, 30c and 32a, 32b, 32c. Electron gun 10 further includes a G3 grid having a G31 lower portion and a G32 upper portion. As used herein, the terms "lower portion" or "lower end" refers to the portion or end of a grid facing in the direction of the low voltage portion of the electron gun, i.e., in the direction of the electron gun's cathodes. The terms "higher portion" or "higher end" refers to the portion or end of the grid facing in the direction of the high voltage portion of the electron gun, i.e., in the direction of the CRTs display screen. The G31 lower portion includes three circular beam passing apertures. The G32 upper portion similarly includes three circular apertures each aligned with a respective aperture in the G31 lower portion. Electron gun 10 further includes a flat, plate-like G4 grid having three circular apertures 38a, 38b and 38c. Finally, electron gun 10 includes a G5 and a G6 grid. The G5 grid includes a G52 lower portion and a G55 upper portion, as well as G53 and G54 intermediate portions disposed between and connected to the respective aforementioned upper and lower portions. The G52 lower portion includes three circular apertures, while the G55 upper portion includes a single chainlink-shaped common beam passing aperture. An inner portion of the G5 grid where the G53 and G54 intermediate portions are in abutting contact also includes three circular beam passing apertures. The G6 grid includes a G61 lower portion and a G62 upper portion. The G61 lower portion includes a chainlink-shaped common aperture in facing relation with the G5 grid, while the G62 upper portion includes three circular apertures. Three cathodes, with only one cathode shown as element K in FIG. 2 for simplicity, direct energetic electrons toward the G1 control grid. Three electron beams, with only one electron beam shown in dotted line form in FIG. 2 for simplicity as element 18, are directed through apertures 12a in a shadow mask 12 and onto a phosphor coating 14 disposed on the inner surface of a CRT display screen 16. The G1 grid is typically maintained at neutral potential, while a V.sub.G2 voltage source 20 (in the range of 300-1000 V) is coupled to the G2 and G4 grids. A V.sub.F voltage source 22 (in the range of 20%-32% of the anode voltage V.sub.A) is coupled to and provides a focus voltage to the G3 and G5 grids, while a V.sub.A voltage source 24 (approximately 25 kV) provides an accelerating voltage to the G6 grid. Cathodes K, the G1 grid, the G2 grid, and the G31 lower portion of the G3 grid comprise a beam forming region (BFR) 26. The G32 upper portion of the G3 grid in combination with the G4 grid and the G52 lower portion of the G5 grid form a prefocus lens 27. The G55 high end of the G5 grid in combination with the G6 grid form the electron gun's main focus lens 28. The facing chainlink-shaped apertures 66 and 68 respectively in the G55 higher end of the G5 grid and in the G61 lower portion of the G6 grid form a main focusing lens in electron gun 10.
The common main focusing lens approach is not without its own unique design considerations and problems. For example, in the common lens approach it is difficult to equalize the focus voltages of the center and two outer electron beams because the center and outer beams pass through different portions of the common lens aperture and experience different focusing effects in both the horizontal and vertical directions. The problem is compounded by the requirement to provide a horizontal and vertical focus voltage to each of the three electron beams. In the past, the parameters of the beam passing aperture in the common lens have been adjusted to compensate for astigmatism and static misconvergence between the outer electron beams and the center electron beam. For example, the width and S height of the common racetrack aperture; the width, height and outer, or end, radii of the dog bone-shaped aperture; and the radius and pitch between the center and outer electron gun diameters in the chain-link aperture common lens are generally adjusted to provide a compromise between beam astigmatism and convergence between the two outer electron beams and the center electron beam. This approach suffers from interaction between the center and outer electron optics focus lenses, making it difficult to achieve the optimum balanced design for both the center and outer electron guns at the same time.
Another prior art approach employing an auxiliary grid disposed next to the common lens and having elliptical beam passing apertures allows for a certain degree of equalizing of the focus voltages of the center and two outer electron beams. By changing the ellipticity of the apertures in the auxiliary grid, limited control over the focus voltages applied to the center and outer electron beams is possible, permitting limited equalization of the focus voltage applied to the three electron beams. However, because there is interaction between the electron optics focus lenses of adjacent electron beams, it is very difficult to compensate for the astigmatism and focus voltage of one electron beam without adversely affecting these same two parameters in an adjacent electron beam.
The present invention addresses the aforementioned limitations of the prior art by applying a compensating electrostatic field to the three electron beams in a portion of the electron gun where there is virtually no overlap, or interaction, between the electron optics lenses of adjacent electron beams. In this region due the very close spacing between grids where there is no overlap, or interaction, between adjacent electron beam's asymmetric electron optics lenses, it is possible to fine tune the electron gun to correct for electron beam astigmatism and static misconvergence to more easily achieve a balanced optimum performance for the center and two outer electron guns. This invention avoids the "cross-talk" effect between adjacent electron optics lenses of adjacent electron beams through the use of asymmetric beam passing apertures each of which includes a center round portion for guiding a beading mandrel to facilitate grid alignment during electron gun assembly and a superimposed elliptically shaped outer portion.